Dual Time Point PET/CT Acquisition Using Ga-68-PSMA-Radioligand

Objectives To evaluate the benefit of dual time point Ga-68-PSMA-PET/CT of the pelvis to differentiate local recurrence and nodal metastases from urinary excretion related activity in bladder and ureters.

Methods 20 consecutive patients with Pca referred for PSMA-PET/CT were included in this analysis. Whole body PET/CT was performed 67±10 minutes after injection of 168.3±18.7 MBq Ga-68-PSMA (DKFZ-Ga-PSMA-11), followed by a late acquisition of the pelvis 123±14 minutes after injection. Contrast enhanced computed tomography of the abdomen and pelvis was performed in 15 patients during the early scan, a low dose CT of the pelvis for attenuation correction was performed during the late scan. Body weight based standardized uptake values were measured in each patient.

Results 73 lesions in 20 patients were analyzed according to uptake changes within the lesions, 10 lesions in the prostate bed, 31 lymph nodes and 32 skeletal lesions. SUVmax of local recurrences, lymph nodes and skeletal lesions increased significantly from early to late scan: 16 vs 20 (p=0,03), 19 vs 22 (p=0,002) and 16 vs 20 (p=0,002). Urinary bladder activity increased and decreased in individual patients without a significant trend, SUVmax early to late scan: 32 vs. 37 (n.s.). If contrast enhanced CT was performed the ureter of both sides could be discriminated easily from the surrounding tissue in all patients. Using this anatomical information, no ambiguous findings needed the late scan for clarification.

Conclusions Due to the high uptake of Ga-68-PSMA and to the variability of urinary activity a second delayed acquisition has no additional benefit. Using the CT-component of the hybrid device helps to overcome ambiguous findings. An additional acquisition should be performed only if needed, e.g. para iliacal focal uptakes to be discriminated from the ureter or para vesicial lesions. In these cases an injection of furosemide must be considered and should be subject of further studies.